Using a scripted expression driver to combine two angles
I assume you mean that you want a "convex combination" of the angles, so that:


0 will return an angle that is just BeginAngle 
0.5 will return an angle halfway between BeginAngle and EndAngle
0.33 will return an angle that is closer to BeginAngle than EndAngle


In that case, you can use the scripted expression:

(1.0 - 0.33) * BeginAngle + (0.33) * EndAngle